1. Technical Field
Exemplary embodiments of this disclosure relate to a transfer device that transfers a toner image on a surface of an image bearing body to a recording material which is nipped in a transfer nip by contact between the image bearing body and a nip formation member, and an image forming apparatus incorporating the transfer device.
2. Description of the Related Art
Image forming apparatuses are used as, for example, copiers, printers, facsimile machines, and multi-functional devices having at least one of the foregoing capabilities. As one type of image forming apparatus, electrophotographic image forming apparatuses are known. Such an electrophotographic image forming apparatus may have a transfer device that transfers a toner image on the surface of an image bearing body to a recording material which is nipped in a transfer nip by contact between the image bearing body and a nip formation member.
For example, an image forming apparatus forms a toner image on the surface of a drum-like photoreceptor by a well-known electrophotographic process. An endless intermediate transfer belt that is provided in a transfer device and serves as an image bearing body is brought into contact with the photoreceptor, thereby forming a primary transfer nip. Then, in the primary transfer nip, the toner image on the photoreceptor is primarily transferred to the intermediate transfer belt.
A secondary transfer roller that is provided in the transfer device and serves as a transfer member is brought into contact with the intermediate transfer belt, thereby forming a secondary transfer nip. A secondary transfer opposed roller is disposed inside a loop of the intermediate transfer belt, and the intermediate transfer belt is nipped between this secondary transfer opposed roller and the secondary transfer roller.
While the secondary transfer roller outside the loop is connected to a ground, a power supply applies a secondary transfer bias to the secondary transfer roller inside the loop. Consequently, a secondary transfer electric field that allows the electrostatic movement of the toner image from the secondary transfer opposed roller side to the secondary transfer roller side is formed between the secondary transfer opposed roller and the secondary transfer roller.
Then, the toner image on the intermediate transfer belt is secondarily transferred to a recording sheet that is fed to the secondary transfer nip at the timing of synchronization with the toner image on the intermediate transfer belt, by the action of the secondary transfer electric field.
In such a configuration, when a sheet that is rich in surface irregularity, such as Japanese paper is used as a recording sheet, a gradation pattern copying the surface irregularity is easily generated in an image. Sufficient toner is not transferred to concave portions on the surface of the sheet, and the image density of the concave portions is thinner than that of convex portions, thereby causing this gradation pattern.
In the image forming apparatus, a DC power supply and an AC power supply are connected to each other, and the secondary transfer bias including a DC voltage and an AC voltage is applied. For the image forming apparatus, toner reciprocates between the surface concave portions of the recording material and the image bearing body by using such a secondary transfer bias, so that the toner can come into contact with the surface concave portions of the recording material. Consequently, it is possible to suppress the transfer failure of the toner to the surface concave portions of the recording material.
Additionally, it is known that the generation of the gradation pattern can be suppressed by the application of such a secondary transfer bias, compared to a case where a secondary transfer bias configured from only a DC voltage is applied.
As a result of study by the inventors of the present application, the inventors has found that a problem arises that the deterioration of a transfer member or the like, which is not generated by the secondary transfer bias that is configured from only a DC voltage and does not include an AC voltage, is generated, and the life of the member is shortened, in the case where a transfer bias that includes a DC voltage and an AC voltage is used as the secondary transfer bias.